(1) Field of the Invention
The present invention relates to a distributed feed-back laser diode module containing an integrated modulator in which an optical modulator is used to modulate light output from a semiconductor laser diode so that output light has a stable wavelength and an optical semiconductor device using the same. More particularly, this invention is related to an optical semiconductor device capable producing light with a stable wavelength.
(2) Description of the Related Art
In an optical communication system, light output from a semiconductor laser diode is modulated by applying a drive signal, and the modulated light is input into an optical fiber. The light signal is detected at the opposite end of the optical fiber.
Optical communication can provide a high transmission speed because of the high frequency of light itself, and the excellent monochromaticity of light generated by a semiconductor laser diode; that is, the excellent stability in frequency of the light. Nevertheless, there is a demand for a higher transmission speed. In typical digital optical communication, the longer the transmission distance is, the more critical the monochromaticity of the light becomes. For optical communication, therefore, a distributed feedback (DFB) type semiconductor laser diode, which includes a diffraction grating has been employed.
However, when the light from a semiconductor laser diode is modulated by a digital signal, the actual light output fluctuates. Therefore, even when a semiconductor laser diode having a DFB structure is employed, the wavelength characteristic of the light changes. When the wavelength extends, transmission time differs in optical fibers due to the dispersion characteristics of the optical fibers. The transmittable signal frequency is restricted accordingly.
In an effort to minimize the foregoing extension in wavelength of the output light, an optical semiconductor device in which a modulator is provided to modulate light output of the semiconductor laser diode has been proposed. In this optical semiconductor device, since current flowing through the semiconductor laser diode is constant, the semiconductor laser diode emits constant output light whose wavelength does not extend. The output light is emitted into the optical modulator adjoining the semiconductor laser diode. The optical modulator transmits the input light in a normal state, but it does not transmit the input light when a reverse voltage is applied to P-N junction of the modulator. This type of optical modulator is called an electroabsorption modulator, and this type of optical semiconductor device is called a Modulator Integrated Distributed Feedback Laser Diode (MI-DFB-LD).
The present invention is applied to this type of optical semiconductor device.
This type optical semiconductor device is realized by two manufacturing methods. In one method, the semiconductor laser diode and the optical modulator are formed on a same semiconductor substrate which operates as the ground electrode of the semiconductor laser diode and of the optical modulator. In the other method, the semiconductor laser diode and the optical modulator are independently formed, then they are arranged on a conductive base element so that ground electrodes on the diode and the modulator are adhered to the conductive base element and their optical axes coincide each other. Alternately, in the optical semiconductor device, the semiconductor laser diode and the optical modulator are electrically connected via the common conductive element (the semiconductor substrate or the conductive base element). The optical semiconductor device is installed in a package, and it is called an optical module.
The semiconductor laser diode is susceptible to temperature. The intensity and wavelength of light output from the semiconductor laser diode change with temperature. To suppress this influence of temperature, the integrated optical device is encapsulated in a temperature controller for stabilizing the temperature. For example, a Peltier element is used as the temperature controller. In this case, a portion including the MI-DFB-LD must be arrange so that thermal transfer to or from surrounding devices is minimized. Generally speaking, electroconductive material is also heat-conductive. Therefore, the common conductive element shared by the semiconductor laser diode and the optical modulator is therefore connected to a ground terminal of the optical module via an electrical conductor having a small thermal conductivity such as a narrow metallic wire or a conductive bridge. From the electrical aspect, this electric conductor acts as an impedance. Namely, this means that the semiconductor laser diode and the optical modulator are connected to a ground terminal of the optical module via an impedance. Particularly, the impedance appearing between the common conductive element and ground is relatively large at a high frequency.
In order to drive the optical modulator, a drive signal which changes between a positive voltage and a negative voltage at a very high frequency is applied to the optical modulator. This drive signal is conveyed to the common conductive element through the optical modulator. If the common conductive element is connected to ground via a low impedance, the high frequency signal appearing at the common conductive element does not influence the current through the semiconductor laser diode. However, as described above, the impedance between the common conductive element and ground is comparatively large, therefore, the current through the semiconductor laser diode fluctuates in response to the high frequency signal appearing at the common conductive element. If this high frequency signal passes the semiconductor laser diode, an intensity of the light generated by the semiconductor laser diode fluctuates.
Further, when a reverse voltage is applied to the optical modulator, light absorption current abruptly grows. Accordingly, when a drive signal is applied to the optical modulator, a current through the optical modulator abruptly varies. Due to this current variation, a high frequency signal appears at the common conductive element. When the impedance between the common conductive element and ground is comparatively large, this high-frequency signal also causes a fluctuation in the current through the semiconductor laser diode.
In this way, the drive signal affects the semiconductor laser diode.
As a result, the light output intensity of the semiconductor laser diode fluctuates, and the wavelength of the output light changes. Eventually, the transmission speed and distance are reduced.
This problem occurs when common ground electrodes of the semiconductor laser diode and the optical modulator are connected to ground via an element having a large impedance.
Therefore, for example, although the element with an impedance does not exist in the optical module, the same problem will occur when the impedance of an element connected between a ground pin of the optical module and ground is large.
Further, when a signal reflection element which reflects a signal is arranged in place of the impedance element or the signal reflection element is arranged with the impedance element, the same problem will occur.
Namely, when the semiconductor laser diode and the optical modulator are connected to ground via a common impedance element or a signal reflection element, the above-mentioned problem will occur.